Boiling water reactors (BWRs) operating over long periods of time have stainless steel components which are subject to IGSCC. The injection of hydrogen into the feedwater of BWRs has been demonstrated as an effective means of suppressing the stress corrosion cracking of these stainless steel components. Under normal water chemistry conditions, the oxygen concentration is approximately 200 parts per billion (ppb) and the hydrogen concentration is approximately 10 ppb. Under hydrogen water chemistry conditions, the concentrations necessary to prevent ISGCC are in the range of 2-15 ppb oxygen and 100 ppb hydrogen. These concentrations are approximate and vary among reactors.
To inspect for the presence of stress corrosion, non-destructive testing is used. Such non-destructive testing of piping joints requires plant shutdown while the inspection occurs. Thus, even the threat of IGSCC costs the plant expensive down time.
Unfortunately, and coincident with this hydrogen treatment, higher levels of radiation in the main steam lines and turbines have been observed. These higher levels of radiation in the more heavily shielded plants have not caused a significant problem. Heavily shielded turbines, condensers and steam piping have prevented the radiation from finding its way through to operating personnel and occupied areas. Unfortunately, many plants include heavy shielding in the turbine, condenser and steam piping side of the plant which is only adequate to limit dose rates under normal water chemistry. This being the case, the increased levels of radiation have tended to limit the use of hydrogen water chemistry to prevent stress corrosion.
BWR operation under normal water chemistry produces a small fraction of N-16 formed by the n,p reaction of 0-16 and exists in a chemical form which tends to be volatile. As this fraction is transported in the aqueous phase in the reactor and the water coolant is converted to steam, a portion of the volatile fraction is swept into the steam phase and transported to the turbine.
N-16 is a radioactive nuclide whose half-life is approximately 7 seconds. In its decay, high energy gamma radiation of 6 and 7 MeV is emitted. Thus, during normal plant operation a significant radiation field emanates from the steam lines and turbine. Because of the intensity and relatively high energy of the gamma radiation, significant shielding is required to limit the radiation field intensity. In spite of the shielding, the influence of the N-16 source can be measured even at significant distances from the source.
We have discovered as a part of the present invention that the observed radiation levels are caused at least in part by the N-16 being converted into volatile nitrogen compounds, including ammonia, which are transported in the steam phase.
Under hydrogen water chemistry conditions, a larger fraction of the N-16 is converted to a volatile form. Thus, the radiation levels in the steam phase increase significantly when compared to the levels without hydrogen addition. Dependent on the reactor, the levels have been measured by us to increase from 1.2 to 5 times at the hydrogen concentration necessary to prevent IGSCC in the recirculation system. For some plants the increase is sufficient to exceed safety dose rate limits not only close to the source, but also in surrounding buildings and grounds and at site boundaries. This is perceived as one of the most detrimental aspects of hydrogen water chemistry. Thus, it would be highly desirable if a method could be found to limit the N-16 volatility, i.e., the quantity transported to the steam.